glsl: avoid linker and user varying location to overlap

glsl: avoid linker and user varying location to overlap

Current behavior on the interface matching:

layout (location = 0) out0; // Assigned to VARYING_SLOT_VAR0 by user
out1; // Assigned to VARYING_SLOT_VAR0 by the linker

New behavior on the interface matching:

layout (location = 0) out0; // Assigned to VARYING_SLOT_VAR0 by user
out1; // Assigned to VARYING_SLOT_VAR1 by the linker

v4:
* Fix variable name in assert

Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>